netdev - Re: [PATCHv2 dontapply] vhost-net tx tuning

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Date:	Tue, 01 Feb 2011 15:07:38 -0800
From:	Sridhar Samudrala <sri@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Steve Dobbelstein <steved@...ibm.com>, mashirle@...ux.vnet.ibm.com,
	kvm@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCHv2 dontapply] vhost-net tx tuning

On Tue, 2011-02-01 at 17:52 +0200, Michael S. Tsirkin wrote:
> OK, so thinking about it more, maybe the issue is this:
> tx becomes full. We process one request and interrupt the guest,
> then it adds one request and the queue is full again.
> 
> Maybe the following will help it stabilize?  By default with it we will
> only interrupt when we see an empty ring.
> Which is liklely too much: pls try other values
> in the middle: e.g. make bufs half the ring,
> or bytes some small value like half ring * 200, or packets some
> small value etc.
> 
> Set any one parameter to 0 to get current
> behaviour (interrupt immediately when enabled).
> 
> Warning: completely untested.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> 
> ---
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index aac05bc..6769cdc 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -32,6 +32,13 @@
>   * Using this limit prevents one virtqueue from starving others. */
>  #define VHOST_NET_WEIGHT 0x80000
> 
> +int tx_bytes_coalesce = 1000000000;
> +module_param(tx_bytes_coalesce, int, 0644);
> +int tx_bufs_coalesce = 1000000000;
> +module_param(tx_bufs_coalesce, int, 0644);
> +int tx_packets_coalesce = 1000000000;
> +module_param(tx_packets_coalesce, int, 0644);
> +
>  enum {
>  	VHOST_NET_VQ_RX = 0,
>  	VHOST_NET_VQ_TX = 1,
> @@ -127,6 +134,9 @@ static void handle_tx(struct vhost_net *net)
>  	int err, wmem;
>  	size_t hdr_size;
>  	struct socket *sock;
> +	int bytes_coalesced = 0;
> +	int bufs_coalesced = 0;
> +	int packets_coalesced = 0;
> 
>  	/* TODO: check that we are running from vhost_worker? */
>  	sock = rcu_dereference_check(vq->private_data, 1);
> @@ -196,14 +206,26 @@ static void handle_tx(struct vhost_net *net)
>  		if (err != len)
>  			pr_debug("Truncated TX packet: "
>  				 " len %d != %zd\n", err, len);
> -		vhost_add_used_and_signal(&net->dev, vq, head, 0);
>  		total_len += len;
> +		packets_coalesced += 1;
> +		bytes_coalesced += len;
> +		bufs_coalesced += out;
> +		if (unlikely(packets_coalesced > tx_packets_coalesce ||
> +			     bytes_coalesced > tx_bytes_coalesce ||
> +			     bufs_coalesced > tx_bufs_coalesce))
> +			vhost_add_used_and_signal(&net->dev, vq, head, 0);

I think the counters that exceed the limits need to be reset to 0 here.
Otherwise we keep signaling for every buffer once we hit this condition.

Thanks
Sridhar

> +		else
> +			vhost_add_used(vq, head, 0);
>  		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>  			vhost_poll_queue(&vq->poll);
>  			break;
>  		}
>  	}
> 
> +	if (likely(packets_coalesced &&
> +		   bytes_coalesced &&
> +		   bufs_coalesced))
> +		vhost_signal(&net->dev, vq);
>  	mutex_unlock(&vq->mutex);
>  }
> 
> --
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